The size of the hard fiber washer (MG#13934700) used to seal the bolt that secures the fuel inlet pipe to the carburetor body is approximately 6 mmID × 10 mmOD × 1 mm thick.

Dan Eberhardt reports that McMaster-Carr part number 95601A310 will work well (ID = 1⁄4 inch and OD = 3⁄8 inch hard fiber washer). Thanks to Dan Eberhardt for sending me this information in a private communication.